This invention relates to an apparatus and method for discriminating between colors of an object by detecting the energy of the light reflected from the object, and more particularly to a sensor for heightening the color discrimination ability.
A method of operating a sorting machine by means of a sensor which detects the color of a color mark which is present on an article has recently been adopted for the purpose of, for example, automatization of an article sorting operation. FIG. 1 shows the structure of such a conventional color mark sensor consisting of a light source 1 for radiating white light of predetermiend spectral energy, a lens 2 for converging the light which is radiated from the light source 1 and for projecting the light onto a half mirror 3, a lens 4 for converging the light which is projected from the lens 2 onto the half mirror 3 and for projecting it onto an object 5, and lens 6 for converging the reflected light of the light which is projected onto the object 5 in the above-described way after successively passing the light through the lens 4 and the half mirror 3 and for projecting it onto a visual sensitivity correction filter 7, which has a spectral transmittance approximately corresponding to its spectral luminous efficiency. A silicon photodiode 8 is used for receiving the light which has passed through the filter 7, an amplifier 9 is used for amplifying an output signal 8a from the silicon photodiode 8, and an output signal 9a is transmitted from the amplifier 9. Since the sensor shown in FIG. 1 has the above-described structure, the light which enters the phototdiode 8 through the filter 7 has a spectral energy distribution which is approximately the same as the human eye capable of recognizing the color of the object 5. The photodiode 8 detects the energy of each wavelength component in accordance with the spectral sensitivity of the diode 8, and converts the total amount of the detected energy to an electrical signal 8a. In other words, the amplified signal 9a is a signal corresponding to the total amount of the energy which is detected by the photodiode 8, and in this sensor, the color of the object 5 is identified on the basis of this signal 9a.
The principle of color discrimination on the basis of the signal 9a shown in FIG. 1 is next explained with reference to FIG. 2. In FIG. 2, the curves in the solid lines show the spectral reflectances of objects having the respective indicated colors, and the curve Z in the dotted line shows the product of the spectral transmittance of the filter 7 and the spectral sensitivity of the photodiode 8 in FIG. 1. In the sensor shown in FIG. 1, since the object 5 is irradiated by the white light source 1, the spectral energy distribution of the light reflected from the object 5 is equal to the distribution indicated by the corresponding solid curve for the relevant color in FIG. 2. Since the reflected light is detected by the photodiode 8, as described above, the total light energy of the color of the object which is detected by the diode 8 takes the value equivalent to the value of the area which is defined by a curve obtaind by multiplying the spectral reflectance of the object by the value of the curve Z in accordance with the curve for the relevant color and the abscissa in FIG. 2. Accordingly, if the energy of light detected by the photodiode 8 is different in correspondence with the color of an object, the color is identified by the signal 9a.
The characteristic curve B in FIG. 4 shows an example of the results of the experiments carried out by using the sensor shown in FIG. 1. The Figure shows the relative values of the output signal 9a of the amplifier 9 in relation to the objects of the respective indicated colors. As is evident from the Figure, the output signal 9a shows the different values which depend upon the color, and discrimination of the five colors other than green is possible by the output signals of the amplifier. However, the difference between the output signals for red, violet, and blue is too small for discrimination between these colors. In addition, since the output signal for green is approximately the same as tht for violet, discrimination between red, violet, blue and green is very difficult.
As is clear from the above description, color discrimination by means of the conventional color mark sensor is difficult in at least four colors, namely, red, violet, blue and green.
Accordingly it is an object of the invention of the parent application to solve the above-described problems in the conventional method of determining the color of an object by detecting the energy of the reflected light from the object, and to provide a method of discriminating between many colors with good sensitivity.
To achieve this aim, a color discriminating method and sensor for determining the color of a particular object is provided, which comprises a transmitting unit for radiating light, focusing lenses and reflecting surfaces for converging, projecting and reflecting the light, one or more sharp cut filters which receive the light for spectral transmittance, and a photodiode for measuring spectral sensitivity. The photodiode converts the light to an electrical signal, which is delivered to an amplifier that amplifies the signal to produce signals discriminated according to the various colors of the sensed object. In our parent application the light energy, which is converted to the electrical signal by the photodiode, is determined by a spectral energy distribution of the light within a range between a lower limit wavelength of about 540 nm to 580 nm and an upper limit wavelength of at least 950 nm.
However, while this technique has proven quite satisfactory for glossy samples, it has proven less satisfactory for non-glossy samples.
The spectral reflectance characteristics of object color are shown in FIG., 2 using a glossy sample and in FIG. 3 using a non-glossy sample, using reference color paper of the Practical Color Coordinate System issued by the Japan Color Laboratories. As shown, in these figures, even if the object colors are the same with respect to the reference color papers, the spectral reflectance characteristics are different in accordance with the surface condition of the object and the color temperature of the light source used for lighting.
In particuar although the glossy sample shown in FIG. 2 brought about a satisfactory characteristic as shown by the curve A in FIG. 4, the non-glossy sample shown in FIG. 3 showed a non-functional characteristic as indicated by curve C of FIG. 4. In particular, it has been found that difference between output signals of the amplifier for respective colors of white, yellow and orange is small and therefore it is considerably difficult to discriminate between these three colors. Although it is obvious from a comparison of FIG. 2 and FIG. 3, the peak values are almost the same at the wavelength of 600 nm or more in the yellow and orange for the non-glossy sample. Therefore, in the method of the parent patent application wherein the lower limit was a wavelength of 540 nm to 580 nm, the energy of wavelengtgh about 540 nm for the yellow component is partially cut out and discrimination is no longer possible. Although not shown, white has almost a flat spectral reflectance characteristic in the total range of wavelength. In the case of non-glossy samples, those tested have the reflectance of about 75% which is equal to N9.0 and it is difficult to discriminate between yellow and orange.
Accordingly, the method proposed in the parent application cannot adequately discriminate colors for the non-glossy sample and therefore an improved method would be desirable.
Accordingly, it is an object of the present invention to solve the above-described problems when determining the color of an object by detecting the energy of the reflected light from the object and to provide a method of discriminating between more colors with high sensitivity.